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I have a Ducane 92% high eff 70,000 BTU furnace that is less than a year old. The contractor and I have been trying to solve an intermittent issue for the last 6 weeks. Hopefully one of you pros might see what we are missing.

This same LED code seems to be used for:
Pressure switch [ condensate drain or combustion air]
Inducer or Inducer Blower Switch

So far, we have tried the following:
- The pressure switch has been changed and tested.
- We have run the air inlet off, to allow the furnace access to combustion air both from the air inlet and room, to ensure lack of combustion air was not an issue.
- The condensate drain has been cleaned, and the drain line is clean.
- The IBS has been changed, and temp at the outlet vent is below 120F, and switch is rated at 140. [Oh, by the way, the differential is 54 F from intake to outlet, with a 40-70 rating.] It is not the temp limit that is cycling, so this shouldn't be involved, but just while temp is being brought up.
- I checked that the furnace was level and not leaning back, to ensure condensate was not pooling in the inducer somewhere, and that this was causing the cycling.

I had a voltmeter on the IBS once, and when the furnace was running normally it was 120V AC either side. [expected] It did cycle flame off while I was watching this. I could swear that I got 120V AC on the Smartvalve side of the IBS, and 0V on the inducer blower side. The LED stayed bright/dim for 30 secs, then started O3 error code. When the code started flashing, the Smartvalve side of the IBS went to 6.4 V AC. BUT..... the IBS switch was changed, and it still cycles!!!!!! I have not been able to measure voltages right when it cycles to catch new data, but have run downstairs when the furnace cycles, and seen the 03 LED error code.

So far, my contractor has been great, and he has not charged me for the switches, condensate trap clean, etc. But this keeps going on and on, and both he and I are getting frustrated with the furnace and each other. Any help would be greatly appreciated!

Replace the circut board, some problems were on the ducane units. Look at the back of the board, see any brown spots like it has gotten hot? How about a model # and serial # this really helps. Also note this is not a d.i.y. for the homeowner to replace a board,also could be the smart valve. they have settings and need to be installed by a pro contractor. If he cant figure it out then it is time to look for one whom knows what they are doing. Also does it work properly when you turn the power off and restart the unit?

Sounds like a parts changer not a service tech. Unless I missed it, I see nowhere mentioning what the draft reading is vs what the switch is looking for. Put a magnahelic or manometer on the vacuum tubing and see what the system is drawing. Most of the time, the pressure switch is doing its job and doesn't need changing (right Doc and jrbenny!) but gets changed anyway because it opens up. If your actual reading is right at pressure switch make/break point, somebody better figure out why. Undersized pipes, sag causing blockage in the flue, something.

I have read Boss' posting on rules, and do know this is not a DIY site. I am working with a service tech who installed this furnace and continues to work with me, but my situation is a little uncomfortable in that he is a personal friend as well. He put in the furnace for me at an exceptionally reasonable price out of friendship. So, I am stuck now, not wanting to suggest he isn't performing, and wanting to see the furnace finally work properly. He has been checking temps and replacing switches, but everytime I brought up a pressure check, he has stated everything is OK because all the lengths are short on the outlet, and we proved combustion air [inlet] wasn't a problem because taking the air inlet off didn't cure the cycling. I have tried to ask him to check what the manual lists as "external static" pressure, but again his response was basically " the temp rise is OK so the airflow is OK". [As I mentioned before, the temp rise input plenum to output plenum is 54 F, with a 40-70 rating.]

I really do appreciate your responses, and hope you will continue to help. I can then go to Bernie with a list of things to check all in one Saturday, instead of the change a part and wait a week cycle we are in now. And from your responses, he seems to have an aversion to manometers! 8>)

More info below per your responses:

Model CMPE075U3
SN 2848870242
20537404

If I cycle the power, the furnace does seem to work properly. But, on at least a couple of occasions, when I went down to check after hearing it cycle off, the furnace has been in lockout [6 3], so it has failed 4 times in one call for heat.

The outlet length is only 18 feet, plus 6 elbows [ 4- 45's and 2 90's ]. 2 inch concentric vent kit- [Bernie checked outlet flue temp when he replaced the IBS. It was 120F, and the IBS switch is rated at 140F.] So, even though I was sure the IBS went open from my voltage measurements [first message], I am pretty sure he doesn't believe me. And by now, I am not sure I am sure myself! 8>)

Pressure switch was changed, but he did not do a manometer check of actual pressure.

So, I plan to push him to bring a manometer next time he comes. If nothing pans out for outlet flue pressure, then
I will ask him to replace the circuit board as 9mmnow suggests.

And while I am playing with the thermostat to see if it is still cycling after he replaces something, I am going to take my family and dogs out for a walk if he ends up needing to replace the Smartvalve. I am only involved in this whole situation because of the friendship issue.

Don't go replacing boards or anything til you find out what the problem is. You'd think his distributor would be tired of writing warranty credits on a whole furnace If you are getting a code that the pressure switch is opening, find out why. A bad blower board won't cause that unless it stops the inducer when it shouldn't. A problem in the venting or drain will. It has nothing to do with temp rise (the limit handles this) only vacuum in the flue & drain system. The flue should be down hill from the termination to the furnace, any sags will trap water and block exhaust.

What is a IBS? Sounds like some kind of high limit on the inducer monitoring flue temp? Put your volt meter on either side of the switch. If closed, no voltage. If it opens, you get 24v. Easiest way to check to see what is opening.

This is two stage furnace, are the switches correct? How about how it is wired? Is it wired for the two stage with a two stage stat or is it jumpered? I have seen many boards burnt on this unit. That is why i said board. Check your wiring on the stages, let us know how it is wired. the board and to the stat and what kind of stat.This kind of sounds like it is going out on second stage

Cause known, working on solution

Thanks for all of the advice. I know why it is cycling, but need to work with my buddy to try to get it solved.

I borrowed a logging multimeter from work, to check the switches. [It records min and max voltages, for up to 36 hours.]

I put this on the IBS [Inducer Blower Switch] overnight. The switch is in-line [electrically in series] with the inducer blower motor. This switch is a thermal, normally closed, and goes open at 140 F. When normally closed, the switch has under a volt AC on it. In the morning though, the meter showed that the switch had 120V AC on it at some point, proving that it went open.

From what loonie and aircon said, I think the problem is not enough angle on the outlet flue. He worked things around well in a tight furnace room to use the existing hole in the wall, but the furnace is only 12 feet from the wall, and the run takes a jog around things, for 18 feet total, plus 6 elbows [ 4- 45's and 2 90's ] into a 2 inch concentric vent kit. There is a 6 foot section that runs pretty much level. So, I plan to ask him to redo the outlet flue, putting more angle back to the furnace.
Per 'beenthere's question, there is no drip tee.

From what you all have suggested, and from reading the install guide, it says condensate collecting in the flue will block exhaust flow. This would explain why sometimes it cycles at 12 minutes, sometimes closer to 20, sometimes not for much longer. It also explains why Bernie wouldn't have seen this, because he drops by late afternoon, when the humidity in the air is lower. The worst cycling occurs in early morning, on recovery from setback, and when early morning moisture is high. But a question, why can't the inducer blower overcome any water vapour that the condensate creates though????

I've seen other units do very simular to yours, when they have a almost straight run, and or no drip tee.
The moisture is put into the flue pipe little by little, but collects in an area where it is now a larger volume, and with the low pressure from the inducer it just stays in that area till it restricts the flue enough to either over heat the safety, or the pressure switch opens.

Do as you suggested with reangling the flue and have a drip tee installed.